Silicon-based energy storage devices with electrolyte containing dihydrofuranone based compound

ABSTRACT

Electrolytes and electrolyte additives for energy storage devices comprising dihydrofuranone based compounds are disclosed. The energy storage device comprises a first electrode and a second electrode, wherein at least one of the first electrode and the second electrode is a Si-based electrode, a separator between the first electrode and the second electrode, an electrolyte comprising at least two electrolyte co-solvents, wherein at least one electrolyte co-solvent comprises a dihydrofuranone based compound.

BACKGROUND Field

The present application relates generally to electrolytes for energystorage devices. In particular, the present application relates toelectrolytes and additives for use in lithium-ion energy storage deviceswith silicon-based anode materials.

Description of the Related Art

As the demands for both zero-emission electric vehicles and grid-basedenergy storage systems increase, lower costs, high energy density, highpower density, and safety of energy storage devices, such as lithium(Li)-ion batteries, are highly desirable. Improving the energy and powerdensity and the safety of Li-ion batteries requires the development ofhigh-capacity high-voltage cathodes, high-capacity anodes, andaccordingly, functional electrolytes with high voltage stability andinterfacial compatibility with electrodes.

A Li-ion battery typically includes a separator and/or electrolytebetween an anode and a cathode. In one class of the battery, theseparator, cathode and anode materials are individually formed intosheets or films. Sheets of the cathode, separator and anode aresubsequently stacked or rolled with the separator separating the cathodeand the anode (collectively, electrodes) to form a battery. A typicalelectrode includes electro-chemically active material layer on anelectrically conductive metal (e.g., aluminum or copper). Films can berolled, or cut into pieces which are then layered into stacks. The stackincludes alternating electro-chemically active materials with theseparator between them.

Silicon (Si) is one of the most promising anode materials for Li-ionbatteries due to its high specific gravimetric and volumetric capacity(3579 mAh/g and 2194 mAh/cm³ vs. 372 mAh/g and 719 mAh/cm³ forgraphite), and low lithiation potential (<0.4 V vs. Li/Li⁺). Among thevarious cathodes presently available, layered lithium transition-metaloxides such as Ni-rich Li[Ni_(x)Co_(y)Mn(Al)_(1-x-y)]O₂ (NCM or NCA) arethe most promising cathodes due to their high theoretical capacity (˜280mAh/g) and relatively high average operating potential (3.6 V vsLi/Li⁺). In addition to Ni-rich NCM or NCA cathode, LiCoO₂ (LCO) is alsoa very attractive cathode material because of its relatively hightheoretical specific capacity of 274 mAh g⁻¹, high theoreticalvolumetric capacity of 1363 mAh cm⁻³, low self-discharge, high dischargevoltage, and good cycling performance. Coupling Si-based anodes withhigh-voltage Ni-rich NCM (or NCA) or LCO cathodes can deliver moreenergy than conventional Li-ion batteries with graphite-based anodes,due to the high capacity of these new electrodes. However, both Si-basedanodes and high-voltage Ni-rich NCM (or NCA) or LCO cathodes faceformidable technological challenges, and long-term cycling stabilitywith high-Si anodes paired with NCM or NCA cathodes has yet to beachieved.

For anodes, Si-based materials can provide significant improvement inenergy density. However, the large volumetric expansion (>300%) duringthe Li alloying/de-alloying processes can lead to disintegration of theactive material and the loss of electrical conduction paths, therebyreducing the cycling life of the battery. In addition, an unstable solidelectrolyte interphase (SEI) layer can develop on the surface of thecycled anodes. As the active material expands and contracts during eachcharge-discharge cycle, unreacted Si surfaces in the active material cansubsequently be exposed to the liquid electrolyte and form thicker SEIlayers. This results in an irreversible capacity loss at each cycle dueto the reduction at the low potential where the liquid electrolytereacts with the exposed unreacted surface of the Si in the anode. Inaddition, oxidative instability of the conventional non-aqueouselectrolyte takes place at voltages beyond 4.5 V, which can lead toaccelerated decay of cycling performance. Because of the generallyinferior cycle life of Si compared to graphite, only a small amount ofSi or Si alloy is used in conventional anode materials.

The NCM (or NCA) or LCO cathode usually suffers from an inferiorstability and a low capacity retention at a high cut-off potential. Thereasons can be ascribed to the unstable surface layer's gradualexfoliation, the continuous electrolyte decomposition, and thetransition metal ion dissolution into electrolyte solution. The majorlimitations for LCO cathode are high cost, low thermal stability, andfast capacity fade at high current rates or during deep cycling. LCOcathodes are expensive because of the high cost of Co. Low thermalstability refers to exothermic release of oxygen when a lithium metaloxide cathode is heated. In order to make good use of Si-basedanode//NCM or NCA cathode and Si-based anode//LCO cathode Li-ion batterysystems, the aforementioned barriers need to be overcome.

The majority of electrolytes for Si anode-based Li-ion batteriesliteratures are carbonate-based solutions with LiPF₆ salt dissolved inthe mixture of cyclic alkyl carbonate (such as ethylene carbonate (EC),vinylene carbonate (VC), propylene carbonate (PC), etc.) and one or morelinear carbonates (such as ethylene methyl carbonate (EMC), dimethylcarbonate (DMC), diethyl carbonate (DEC), etc.) with/without smallamounts of additives. In recent years, fluoroethylene carbonate (FEC)has been frequently used as an additive, co-solvent, or even mainsolvent in the Si anode-based Li-ion batteries. However, highFEC-containing electrolyte formulation-based cells suffer from gasgeneration and volume swelling due to the decomposition of FEC phaseupon prolonged cycling. High FEC-containing electrolyte formulationsalso have a high viscosity which can reduce cell rate performance andthe performance at extreme conditions. In addition, FEC is relativelyexpensive. Thus, different additives and/or co-solvents for Sianode-based.

SUMMARY

In one aspect, an energy storage device is described. The deviceincludes a first electrode, a second electrode, a separator between thefirst electrode and the second electrode, and an electrolyte system. Theelectrolyte system includes a dihydrofuranone based compound, a linearcarbonate, a cyclic carbonate, and a Li-containing salt.

In some embodiments, the dihydrofuranone based compound is of theformula (I):

In some embodiments, R₁ is a C1-C8 alkyl substituted by F. In someembodiments, each R₂, R₃, and R₄ is independently an —H or a C1-C8alkyl.

In some embodiments, the the dihydrofuranone based compound is selectedfrom the group consisting of alpha-(Trifluoromethyl)-gamma-butyrolactoneand γ-Methyl-α-(trifluoromethyl)-γ-valerolactone, or combinationsthereof. In some embodiments, the the linear carbonate is selected fromthe group consisting of ethyl methyl carbonate (EMC), dimethyl carbonate(DMC), and diethyl carbonate (DEC). In some embodiments, the theelectrolyte system further comprises a co-solvent selected from thegroup consisting of methyl acetate (MA), ethyl acetate (EA), methylpropanoate, and gamma butyrolactone (GBL). In some embodiments, the thecyclic carbonate is selected from the group consisting of fluoroethylenecarbonate (FEC), di-fluoroethylene carbonate (DiFEC), Trifluoropropylenecarbonate (TFPC), ethylene carbonate (EC), vinyl carbonate (VC),propylene carbonate (PC), -fluoromethyl-5-methyl-1,3-dioxolan-2-one(F-t-BC), 3,3-difluoropropylene carbonate (DFPC), and3,3,4,4,5,5,6,6,6-Nonafluorohexyl-1-ene carbonate. In some embodiments,the cyclic carbonate is a fluorine containing cyclic carbonate. In someembodiments, the fluorine containing cyclic carbonate is FEC at aconcentration of about 5% or more. In some embodiments, the electrolyteis substantially free of non-fluorine containing cyclic carbonate.

In some embodiments, the Li-containing salt is a fluorinated Li salt. Insome embodiments, the Li-containing salt has a concentration of about 1M or more. In some embodiments, the at least one of the first electrodeand the second electrode is a Si-base electrode. In some embodiments,the Si-based electrode is an anode. In some embodiments, the anode is aSi-dominant anode. In some embodiments, the anode includes greater than0% and less than about 99% by weight of Si particles, and greater than0% and less than about 90% by weight of one or more types of carbonphases, wherein at least one of the one or more types of carbon phasesis a substantially continuous phase that holds the composite materialfilm together such that the silicon particles are distributed throughoutthe composite material film. In some embodiments, the anode includesgreater than 50% and less than about 97% by weight of Si particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-sectional schematic diagram of an example of alithium-ion battery 300 implemented as a pouch cell.

DETAILED DESCRIPTION Definitions

The term “alkyl” refers to a straight or branched, saturated, aliphaticradical having the number of carbon atoms indicated. The alkyl moietymay be branched or straight chain. For example, C1-C6 alkyl includes,but is not limited to, methyl, ethyl, propyl, isopropyl, butyl,isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, hexyl, etc. Otheralkyl groups include, but are not limited to heptyl, octyl, nonyl,decyl, etc. Alkyl can include any number of carbons, such as 1-2, 1-3,1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10, 1-11, 1-12 2-3, 2-4, 2-5, 2-6, 3-4,3-5, 3-6, 4-5, 4-6 and 5-6. The alkyl group is typically monovalent, butcan be divalent, such as when the alkyl group links two moietiestogether.

The term “fluoro-alkyl” refers to an alkyl group where one, some, or allhydrogen atoms have been replaced by fluorine.

The term “alkylene” refers to an alkyl group, as defined above, linkingat least two other groups, i.e., a divalent hydrocarbon radical. The twomoieties linked to the alkylene can be linked to the same atom ordifferent atoms of the alkylene. For instance, a straight chain alkylenecan be the bivalent radical of —(CH₂)_(n)—, where n is 1, 2, 3, 4, 5, 6,7, 8, 9, or 10. Alkylene groups include, but are not limited to,methylene, ethylene, propylene, isopropylene, butylene, isobutylene,sec-butylene, pentylene and hexylene.

The term “alkoxy” refers to alkyl group having an oxygen atom thateither connects the alkoxy group to the point of attachment or is linkedto two carbons of the alkoxy group. Alkoxy groups include, for example,methoxy, ethoxy, propoxy, iso-propoxy, butoxy, 2-butoxy, iso-butoxy,sec-butoxy, tert-butoxy, pentoxy, hexoxy, etc. The alkoxy groups can befurther substituted with a variety of substituents described within. Forexample, the alkoxy groups can be substituted with halogens to form a“halo-alkoxy” group, or substituted with fluorine to form a“fluoro-alkoxy” group.

The term “alkenyl” refers to either a straight chain or branchedhydrocarbon of 2 to 6 carbon atoms, having at least one double bond.Examples of alkenyl groups include, but are not limited to, vinyl,propenyl, isopropenyl, 1-butenyl, 2-butenyl, isobutenyl, butadienyl,1-pentenyl, 2-pentenyl, isopentenyl, 1,3-pentadienyl, 1,4-pentadienyl,1-hexenyl, 2-hexenyl, 3-hexenyl, 1,3-hexadienyl, 1,4-hexadienyl,1,5-hexadienyl, 2,4-hexadienyl, or 1,3,5-hexatrienyl. Alkenyl groups canalso have from 2 to 3, 2 to 4, 2 to 5, 3 to 4, 3 to 5, 3 to 6, 4 to 5, 4to 6 and 5 to 6 carbons. The alkenyl group is typically monovalent, butcan be divalent, such as when the alkenyl group links two moietiestogether.

The term “alkenylene” refers to an alkenyl group, as defined above,linking at least two other groups, i.e., a divalent hydrocarbon radical.The two moieties linked to the alkenylene can be linked to the same atomor different atoms of the alkenylene. Alkenylene groups include, but arenot limited to, ethenylene, propenylene, isopropenylene, butenylene,isobutenylene, sec-butenylene, pentenylene and hexenylene.

The term “alkynyl” refers to either a straight chain or branchedhydrocarbon of 2 to 6 carbon atoms, having at least one triple bond.Examples of alkynyl groups include, but are not limited to, acetylenyl,propynyl, 1-butynyl, 2-butynyl, isobutynyl, sec-butynyl, butadiynyl,1-pentynyl, 2-pentynyl, isopentynyl, 1,3-pentadiynyl, 1,4-pentadiynyl,1-hexynyl, 2-hexynyl, 3-hexynyl, 1,3-hexadiynyl, 1,4-hexadiynyl,1,5-hexadiynyl, 2,4-hexadiynyl, or 1,3,5-hexatriynyl. Alkynyl groups canalso have from 2 to 3, 2 to 4, 2 to 5, 3 to 4, 3 to 5, 3 to 6, 4 to 5, 4to 6 and 5 to 6 carbons. The alkynyl group is typically monovalent, butcan be divalent, such as when the alkynyl group links two moietiestogether.

The term “alkynylene” refers to an alkynyl group, as defined above,linking at least two other groups, i.e., a divalent hydrocarbon radical.The two moieties linked to the alkynylene can be linked to the same atomor different atoms of the alkynylene. Alkynylene groups include, but arenot limited to, ethynylene, propynylene, butynylene, sec-butynylene,pentynylene and hexynylene.

The term “cycloalkyl” refers to a saturated or partially unsaturated,monocyclic, fused bicyclic, bridged polycyclic, or spiro ring assemblycontaining from 3 to 12, from 3 to 10, or from 3 to 7 ring atoms, or thenumber of atoms indicated. Monocyclic rings include, for example,cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cyclooctyl.Bicyclic and polycyclic rings include, for example, norbornane,decahydronaphthalene and adamantane. For example, C3-C8 cycloalkylincludes cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclooctyl,and norbornane. As used herein, the term “fused” refers to two ringswhich have two atoms and one bond in common. For example, in thefollowing structure, rings A and B are fused

As used herein, the term “bridged polycyclic” refers to compoundswherein the cycloalkyl contains a linkage of one or more atomsconnecting non-adjacent atoms. The following structures

are examples of “bridged” rings. As used herein, the term “spiro” refersto two rings which have one atom in common and the two rings are notlinked by a bridge. Examples of fused cycloalkyl groups aredecahydronaphthalenyl, dodecahydro-1H-phenalenyl andtetradecahydroanthracenyl; examples of bridged cycloalkyl groups arebicyclo[1.1.1]pentyl, adamantanyl, and norbornanyl; and examples ofspiro cycloalkyl groups include spiro[3.3]heptane and spiro[4.5] decane.

The term “cycloalkylene” refers to a cycloalkyl group, as defined above,linking at least two other groups, i.e., a divalent hydrocarbon radical.The two moieties linked to the cycloalkylene can be linked to the sameatom or different atoms of the cycloalkylene. Cycloalkylene groupsinclude, but are not limited to, cyclopropylene, cyclobutylene,cyclopentylene, cyclohexylene, and cyclooctylene.

The term “aryl” refers to a monocyclic or fused bicyclic, tricyclic orgreater, aromatic ring assembly containing 6 to 16 ring carbon atoms.For example, aryl may be phenyl, benzyl or naphthyl, preferably phenyl.Aryl groups may include fused multicyclic ring assemblies wherein onlyone ring in the multicyclic ring assembly is aromatic. Aryl groups canbe mono-, di- or tri-substituted by one, two or three radicals.Preferred as aryl is naphthyl, phenyl or phenyl mono- or disubstitutedby alkoxy, phenyl, halogen, alkyl or trifluoromethyl, especially phenylor phenyl-mono- or disubstituted by alkoxy, halogen or trifluoromethyl,and in particular phenyl.

The term “arylene” refers to an aryl group, as defined above, linking atleast two other groups. The two moieties linked to the arylene arelinked to different atoms of the arylene. Arylene groups include, butare not limited to, phenylene.

The term “heteroaryl” refers to a monocyclic or fused bicyclic ortricyclic aromatic ring assembly containing 5 to 16 ring atoms, wherefrom 1 to 4 of the ring atoms are a heteroatom each N, O or S. Forexample, heteroaryl includes pyridyl, indolyl, indazolyl, quinoxalinyl,quinolinyl, isoquinolinyl, benzothienyl, benzofuranyl, furanyl,pyrrolyl, thiazolyl, benzothiazolyl, oxazolyl, isoxazolyl, triazolyl,tetrazolyl, pyrazolyl, imidazolyl, thienyl, or any other radicalssubstituted, especially mono- or di-substituted, by e.g. alkyl, nitro orhalogen. Pyridyl represents 2-, 3- or 4-pyridyl, advantageously 2- or3-pyridyl. Thienyl represents 2- or 3-thienyl. Quinolinyl representspreferably 2-, 3- or 4-quinolinyl. Isoquinolinyl represents preferably1-, 3- or 4-isoquinolinyl. Benzopyranyl, benzothiopyranyl representspreferably 3-benzopyranyl or 3-benzothiopyranyl, respectively. Thiazolylrepresents preferably 2- or 4-thiazolyl, and most preferred 4-thiazolyl.Triazolyl is preferably 1-, 2- or 5-(1,2,4-triazolyl). Tetrazolyl ispreferably 5-tetrazolyl.

Preferably, heteroaryl is pyridyl, indolyl, quinolinyl, pyrrolyl,thiazolyl, isoxazolyl, triazolyl, tetrazolyl, pyrazolyl, imidazolyl,thienyl, furanyl, benzothiazolyl, benzofuranyl, isoquinolinyl,benzothienyl, oxazolyl, indazolyl, or any of the radicals substituted,especially mono- or di-substituted.

The term “heteroalkyl” refers to an alkyl group having from 1 to 3heteroatoms such as N, O and S. The heteroatoms can also be oxidized,such as, but not limited to, —S(O)— and —S(O)₂—. For example,heteroalkyl can include ethers, thioethers, alkyl-amines andalkyl-thiols.

The term “heteroalkylene” refers to a heteroalkyl group, as definedabove, linking at least two other groups. The two moieties linked to theheteroalkylene can be linked to the same atom or different atoms of theheteroalkylene.

The term “heterocycloalkyl” refers to a ring system having from 3 ringmembers to about 20 ring members and from 1 to about 5 heteroatoms suchas N, O and S. The heteroatoms can also be oxidized, such as, but notlimited to, —S(O)— and —S(O)₂—. For example, heterocycle includes, butis not limited to, tetrahydrofuranyl, tetrahydrothiophenyl, morpholino,pyrrolidinyl, pyrrolinyl, imidazolidinyl, imidazolinyl, pyrazolidinyl,pyrazolinyl, piperazinyl, piperidinyl, indolinyl, quinuclidinyl and1,4-dioxa-8-aza-spiro[4.5]dec-8-yl.

The term “heterocycloalkylene” refers to a heterocyclalkyl group, asdefined above, linking at least two other groups. The two moietieslinked to the heterocycloalkylene can be linked to the same atom ordifferent atoms of the heterocycloalkylene.

The term “pyrolytic carbon” refers to carbon formed by pyrolysis.Pyrolytic carbon may comprise hard carbon and/or soft carbon, but doesnot include graphite.

The term “optionally substituted” is used herein to indicate a moietythat can be unsubstituted or substituted by one or more substituent.When a moiety term is used without specifically indicating assubstituted, the moiety is unsubstituted.

Energy Storage Device

An energy storage device includes a first electrode, a second electrode,a separator between the first electrode and the second electrode, and anelectrolyte in contact with the first electrode, the second electrode,and the separator. The electrolyte serves to facilitate ionic transportbetween the first electrode and the second electrode. One of the firstelectrode and the second electrode is an anode (i.e., negativeelectrode), and the other is a cathode (i.e., positive electrode). Insome embodiments, energy storage devices may include batteries,capacitors, and battery-capacitor hybrids.

In some implementations, at least one electrode may be a Si-basedelectrode. The Si-based electrode may be the anode. In some embodiments,the Si-based anode includes silicon in an amount of about 25% or more ofthe active material used in the electrode. In some embodiments, theSi-based anode is a Si-dominant anode, where silicon is the majority(e.g., in an amount greater than about 50%) of the active material usedin the electrode.

The electrochemical behaviors of Si-based electrodes are stronglydependent on the electrolyte systems, which exert considerably influencenot only on cell safety and kinetics but also on the interfacialproperty including the quality of SEI layer. The properties ofelectrolyte formulations, including Li-containing salt, solvents,additives, etc., are important factors that affect cell energy storage,cycle performance and rate capability (powder density, fast chargingability), etc. To overcome the current obstacles associated withdeveloping high-energy full-cells with Si-based anodes, the nextgeneration of oxidation-stable electrolytes and/or electrolyte additivesare developed. The electrolyte or electrolyte additives can form astable, electronically insulating but ionically conducting SEI layer onthe surface of Si anodes. Additionally, these electrolytes or additivesmay also help modify cathode surfaces, forming stable CEI layers. Thesecould enable the electrochemical stability of Li-ion batteries whencycled at higher voltages and help with calendar life of the batteries.In addition, to alleviate battery safety concerns, these additives mayimpart an increased thermal stability to the organic components of theelectrolyte, drive a rise in the flash point of the electrolyteformulations, increase the flame-retardant effectiveness and enhancethermal stability of SEI or CEI layers on the surface of electrodes.

Electrolyte System

A Li-ion battery includes a first electrode, a second electrode, aseparator between the first electrode and the second electrode, and anelectrolyte in contact with the first electrode, the second electrode,and the separator. The electrolyte serves to facilitate ionic transportbetween the first electrode and the second electrode. In someembodiments, the first electrode and the second electrode can refer toanode and cathode or cathode and anode, respectively. An electrolyte fora Li-ion battery can include at least a solvent and a Li ion source,such as a Li-containing salt. The composition of the electrolyte may beselected to provide a Li-ion battery with improved performance. Forexample, the electrolyte may further contain one or more additionalcomponent(s) such as electrolyte additive(s) and/or co-solvent(s).

As disclosed herein, the electrolyte for a Li-ion battery may include asolvent comprising a cyclic carbonate and/or a linear carbonate. In someimplementations, the cyclic carbonate is a fluorine-containing cycliccarbonate. Examples of the cyclic carbonate include fluoroethylenecarbonate (FEC), di-fluoroethylene carbonate (DiFEC), trifluoropropylenecarbonate (TFPC), ethylene carbonate (EC), vinyl carbonate (VC), andpropylene carbonate (PC), 4-fluoromethyl-5-methyl-1,3-dioxolan-2-one(F-t-BC), 3,3-difluoropropylene carbonate (DFPC),3,3,4,4,5,5,6,6,6-Nonafluorohexyl-1-ene carbonate, etc. Examples of thelinear carbonate include ethyl methyl carbonate (EMC), dimethylcarbonate (DMC), and diethyl carbonate (DEC), and some partially orfully fluorinated ones.

In some implementations, the electrolyte can include more than onesolvent. For example, the electrolyte may include two or moreco-solvents. In some embodiments, at least one of the co-solvents in theelectrolyte is a fluorine-containing compound, such as afluorine-containing cyclic carbonate, a fluorine-containing linearcarbonate, and/or a fluoroether. Examples of fluorine-containingcompound may include FEC, DiFEC, TFPC, F-t-PC, DFPC,1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether,3,3,4,4,5,5,6,6,6-Nonafluorohexyl-1-ene carbonate, and other partiallyor fully fluorinated linear carbonates, partially or fully fluorinatedcyclic carbonates, and partially or fully fluorinated ethers, etc. Insome embodiments, the electrolyte contains FEC. In some embodiments, theelectrolyte contains both EMC and FEC. In some embodiments, theelectrolyte is free or substantially free of non-fluorine-containingcyclic carbonates, such as EC, VC, and PC. In some implementations, theelectrolyte may further contain other co-solvent(s), such as methylacetate (MA), ethyl acetate (EA), methyl propanoate, and gammabutyrolactone (GBL). The cyclic carbonates may be beneficial for SEIlayer formations, while the linear carbonates may be helpful fordissolving Li-containing salt and for Li-ion transport.

An additional component in the electrolyte may be an additive or aco-solvent. As used herein, an additive of the electrolyte refers to acomponent that makes up less than 10% by weight (wt %) of theelectrolyte. In some embodiments, the amount of each additive in theelectrolyte may be from about 0.2 wt % to about 1 wt %, 0.1 wt % toabout 2 wt %, 0.2 wt % to about 9 wt %, from about 0.5 wt % to about 9wt %, from about 1 wt % to about 9 wt %, from about 1 wt % to about 8 wt%, from about 1 wt % to about 8 wt %, from about 1 wt % to about 7 wt %,from about 1 wt % to about 6 wt %, from about 1 wt % to about 5 wt %,from about 2 wt % to about 5 wt %, or any value in between. For example,the total amount of the additive(s) may be from about 1 wt % to about 9wt %, from about 1 wt % to about 8 wt %, from about 1 wt % to about 7 wt%, from about 2 wt % to about 7 wt %, or any value in between.

As used herein, a co-solvent of an electrolyte has a concentration of atleast about 10% by weight (wt %). In some embodiments, a co-solvent ofthe electrolyte may be about 20%, about 40%, about 60%, or about 80%, orabout 90% by weight of the electrolyte. In some embodiments, aco-solvent may have a concentration from about 10% to about 90%, fromabout 10% to about 80%, from about 10% to about 60%, from about 20% toabout 60%, from about 20% to about 50%, from about 30% to about 60%, orfrom about 30% to about 50% by weight.

In the present disclosure, dihydrofuranone based compounds are used asan additional co-solvent or an additive in the electrolyte system forenergy storage devices with Si-based anodes. When used as a co-solventor an additive, the dihydrofuranone based compound can stabilizesolid/electrolyte interface film to reduce electrolyte reactions (e.g.,oxidation on the NCM, NCA, or LCO cathode and reduction on the Si-basedanode), reduce Si-based anode volume expansion, and protect transitionmetal ion dissolution from NCM or NCA cathode and stabilize thesubsequent structure changes. Such co-solvent/additive can also avoidthe exothermic reaction between the released oxygen for LCO and organicelectrolyte and enhance the thermal stability of LCO cathode.Furthermore, such co-solvent/additive can reduce the flammability andenhance the thermal stability of organic electrolytes and increase thesafety of electrolyte solutions. In addition, the presence ofdihydrofuranone based compounds or other proposed chemicals can generateCO₂, which may help form a controlled SEI layer on thesilicon-containing anode by accelerating carbonate-based polymerformation on the surface of the active material, extending the cyclelife. Without being bound by theory, performance improvements may alsobe explained by the fact that CO₂ is a reactive-type additive andthereby may convert reactive ROLi species into ROCO₂Li. Due to theirversatility in reaction chemistry and overall stability inelectrochemical environments, as well as have excellent flame resistanceor fire retardant properties, adding dihydrofuranone based compoundsinto electrolyte solutions may help improve both overall electrochemicalperformance and safety of Si anode-based Li-ion batteries.

An electrolyte system including a dihydrofuranone based compound, alinear carbonate, a cyclic carbonate, and a Li-containing salt isdisclosed. The dihydrofuranone based compound has the following formula:

wherein R₁ is a C1-C8 alkyl substituted by F; and wherein each R₂, R₃,and R₄ is independently an —H or a C1-C8 alkyl.

In some implementations, the dihydrofuranone based compound is selectedfrom alpha-(Trifluoromethyl)-gamma-butyrolactone (CAS 174744-18-4) andγ-Methyl-α-(trifluoromethyl)-γ-valerolactone (CAS 164929-15-1), orcombinations thereof.

Example structures of dihydrofuranone based compounds are shown below:

The concentration of the dihydrofuranone in the electrolyte may be about10% or less, about 0.1% to about 10%, including from about 1% to about5%, and from about 1% to about 3%; about 10% to about 40%, includingfrom about 10% to about 30%, and from about 20% to about 40% by weight;and about 20% to about 100%, including from about 20% to about 80%, andfrom about 30% to about 70% by weight.

A Li-containing salt for a Li-ion battery may include, but not limitedto, lithium hexafluorophosphate (LiPF₆). In some implementations, alithium-containing salt for a Li-ion battery may comprise one or more oflithium tetrafluoroborate (LiBF₄), lithium hexafluoroarsenatemonohydrate (LiAsF₆), lithium perchlorate (LiClO₄), lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI), lithiumbis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB),lithium difluoro(oxalate)borate (LiDFOB), lithium triflate (LiCF₃SO₃),lithium tetrafluorooxalato phosphate (LTFOP), lithium difluorophosphate(LiPO₂F₂), lithium pentafluoroethyltrifluoroborate (LiFAB), and lithium2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI), lithiumbis(2-fluoromalonato)borate (LiBFMB), lithium 4-pyridyl trimethyl borate(LPTB) and lithium 2-fluorophenol trimethyl borate (LFPTB), lithiumcatechol dimethyl borate (LiCDMB), lithium perchlorate (LiClO₄), etc.The electrolyte can have a salt concentration of about 1 moles/L (M) ormore. The electrolyte can also have a salt concentration of about 1.2 M,about 1.5M, or more.

In some implementations, the electrolyte system can include FEC, EMC, adihydrofuranone based compound as disclosed herein, and a Li-containingsalt. The Li-containing salt may be LiPF₆. The concentration of FEC maybe about 5% or more, about 10% or more, from about 20% to about 40%,including about 20%, about 30%, and about 40% by weight. Theconcentration of EMC may be from about 30% to about 60%, including about30%, about 40%, about 50%, and about 60% by weight. The concentration ofa dihydrofuranone based compound may be about 10% or less, from about10% to about 50%, including from about 10% to about 40 vol %, about 10%,about 20%, about 30%, and about 40% by weight.

The electrolyte may include additional additives, such as anoxygen-containing electrolyte additive, a sulfur-containing compounds, afluorine-containing additive, a silicon-containing additive, anitrogen-containing additive, a boron-containing additive, aphosphorus-containing additive, etc. In addition to the heterogeneousatoms, these additives may also contain other functional groups, such asC═C bond, CC bond, ring structures, etc.

In some implementations, the electrolyte system may contain FEC, EMC, adihydrofuranone based compound as disclosed herein, and a Li-containingsalt, without other co-solvent. In some implementations, the electrolytesystem may contain FEC, EMC, a dihydrofuranone based compound asdisclosed herein, and a Li-containing salt, without other additive. Insome implementations, the electrolyte is substantially free ofnon-fluorine containing cyclic carbonate

The electrolyte additives, along with the electrolytes, can be reducedor self-polymerize on the surface of Si-based anode to form a SEI layerthat can reduce or prevent the crack and/or the continuous reduction ofelectrolyte solutions as the Si containing anode expands and contractsduring cycling. Furthermore, these electrolyte additives, along with theelectrolyte solvents, may be oxidized on a cathode surface to form a CEIlayer that can suppress or minimize further decomposition of theelectrolyte on the surface of the cathode. Without being bound to thetheory or mode of operation, it is believed that the presence ofdihydrofuranone based compounds in the electrolyte can result in a SEIand/or CEI layer on the surface of electrodes with improved performance.An SEI layer comprising a dihydrofuranone based compound may demonstrateimproved chemical stability and increased density, for example, comparedto SEI layers formed by electrolytes without additives or withtraditional additives. As such, the change in thickness and surfacereactivity of the interface layer are limited, which may in turnfacilitate reduction in capacity fade and/or generation of excessivegaseous byproducts during operation of the Li-ion battery. A CEI layercomprising a dihydrofuranone based compound may help minimize thetransition metal ion dissolutions and structure changes on cathode sideand may provide favorable kinetics resulting in improved cyclingstability and rate capability. In some embodiments, electrolyte solventscomprising dihydrofuranone based compounds may be less flammable andmore flame retardant.

Electrodes

The cathode for the energy storage device may include Li transitionmetal oxide cathode materials, such as Lithium Cobalt Oxide (LiCoO₂)(LCO), lithium (Li)-rich oxides/layer oxides, nickel (Ni)-richoxide/layered oxides, high-voltage spinel oxides, and high-voltagepolyanionic compounds. Ni-rich oxides/layered oxides may include lithiumnickel cobalt manganese oxide (LiNiCoMnO₂, “NCM”), lithium nickel cobaltaluminum oxide (LiNiCoAlO₂, “NCA”), LiNi_(1−x)M_(x)O₂ andLiNi_(1+x)M_(1−x)O₂ (where M=Co, Mn or Al). Examples of a NCM materialinclude LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ (NCM-622), NCM-111, NC-433,NCM-523, NCM-811, and NCM-9 0.5 0.5. Li-rich oxides/layered oxides mayinclude Li_(y)Ni_(1+x)M_(1−x)O₂ (where y>1, and M=Co, Mn or Ni),xLi₂MnO₃.(1−x)LiNi_(a)Co_(b)Mn_(c)O₂, andxLi₂Mn₃O₂.(1−x)LiNi_(a)Co_(b)Mn_(c)O₂. High-voltage spinel oxides mayinclude lithium manganese spinel (LiMn₂O₄, “LMO”) or lithium nickelmanganese spinel (LiNi_(0.5)Mn_(1.5)O₄, “LNMO”). High-voltagepolyanionic compounds may include phosphates, sulfates, silicates,titanate, etc. One example of polyanionic compound may be lithium ironphosphase (LiFePO₄, “LFP”).

In order to increase volumetric and gravimetric energy density of Li-ionbatteries, silicon may be used as the active material for the anode.Thus, the anode for the energy storage device include Si-based anode.Several types of silicon materials, e.g., silicon nanopowders, siliconnanofibers, porous silicon, and ball-milled silicon, are viablecandidates as active materials for the anode. Alternatively, asdescribed in U.S. patent application Ser. Nos. 13/008,800 and13/601,976, entitled “Composite Materials for Electrochemical Storage”and “Silicon Particles for Battery Electrodes,” a Si-based anode canalso contain a composite material film that includes Si particlesdistributed in a carbon phase. The Si-based anode can include one ormore types of carbon phases. At least one of these carbon phases is asubstantially continuous phase that extends across the entire film andholds the composite material film together. The Si particles aredistributed throughout the composite material film.

The composite material film may be formed by pyrolyzing a mixturecomprising a precursor (such as a polymer or a polymer precursor) and Siparticles. The mixture can optionally further contain graphiteparticles. Pyrolyzation of the precursor forms a pyrolytic carbon andresults in one or more type of carbon phases. In some implementations,the composite material film can have a self-supporting monolithicstructure, and therefore is a self-supporting composite material film.Because the precursor is converted into an electrically conductive andelectrochemically active matrix, the resulting electrode is conductiveenough that, in some cases, a metal foil or mesh current collector canbe omitted or minimized. The converted polymer also acts as an expansionbuffer for Si particles during cycling so that a high cycle life can beachieved. In certain implementations, the resulting electrode is anelectrode that is comprised substantially of active material. Theelectrodes can have a high energy density of between about 500 mAh/g toabout 1200 mAh/g. The composite material film may also be used as acathode active material in some electrochemical couples with additionaladditives.

The amount of carbon obtained from the precursor can be from about 2% toabout 50%, from about 2% to about 40%, from about 2% to about 30%, fromabout 2% to about 25%, or from about 2% to about 20% by weight of thecomposite material. The carbon is obtained through heating acarbon-containing precursor at a temperature sufficient for pyrolysis ofthe precursor to occur, and is thus a pyrolytic carbon. The carbon fromthe precursor can be hard and/or soft carbon. Hard carbon can be acarbon that does not convert into graphite even with heating in excessof 2800 degrees Celsius. Precursors that melt or flow during pyrolysisconvert into soft carbons and/or graphite with sufficient temperatureand/or pressure. The hard carbon phase can be a matrix phase in thecomposite material. The hard carbon can also be embedded in the pores ofthe additives including silicon. The hard carbon may react with some ofthe additives to create some materials at interfaces. For example, theremay be a silicon carbide layer or silicon carbide containing oxygen(Si—C—O) layer between silicon particles and the pyrolytic carbon.Possible pyrolytic carbon precursors can include polyimide (or apolyimide precursor), other aromatic polyimides, phenolic resins, epoxyresins, poly(p-phenylene vinylene) (PPV),poly(p-phenylene-1,3,4-oxadiazole) (POD), benzimidazobenzophenanthrolineladder (BBL) polymer, and other polymers that have a very high meltingpoint or are cross-linked.

The amount of Si particles in the composite material may be betweengreater than 0% and about 90% by weight, between about 20% and about80%, between about 30% and about 80%, or between about 40% and about80%. In some implementations, the amount of Si particles in thecomposite material may be between about 50% and about 90% by weight,between about 50% and about 80%, or between about 50% and about 70%, andsuch anode is considered as a Si-dominant anode. The amount of one ormore types of carbon phases in the composite material may be betweengreater than 0% and about 90% by weight or between about 1% and about70% by weight. The pyrolyzed/carbonized polymer can form a substantiallycontinuous conductive carbon phase in the entire electrode as opposed toparticulate carbon suspended in a non-conductive binder in one class ofconventional lithium-ion battery electrodes.

The largest dimension of the silicon particles can be less than about 40μm, less than about 1 μm, between about 10 nm and about 40 μm, betweenabout 10 nm and about 1 μm, less than about 500 nm, less than about 100nm, and about 100 nm. All, substantially all, or at least some of thesilicon particles may comprise the largest dimension described above.For example, an average or median largest dimension of the siliconparticles can be less than about 40 μm, less than about 1 μm, betweenabout 10 nm and about 40 μm, between about 10 nm and about 1 μm, lessthan about 500 nm, less than about 100 nm, and about 100 nm.Furthermore, the silicon particles may or may not be pure silicon. Forexample, the silicon particles may be substantially silicon or may be asilicon alloy. The silicon alloy includes silicon as the primaryconstituent along with one or more other elements.

Micron-sized silicon particles can provide good volumetric andgravimetric energy density combined with good cycle life. In certainimplementations, to obtain the benefits of both micron-sized siliconparticles (e.g., high energy density) and nanometer-sized siliconparticles (e.g., good cycle behavior), silicon particles can have anaverage particle size in the micron range and a surface includingnanometer-sized features. The silicon particles have an average particlesize (e.g., average diameter or average largest dimension) between about0.1 μm and about 30 μm or between about 0.1 μm and all values up toabout 30 μm. For example, the silicon particles can have an averageparticle size between about 0.5 μm and about 25 μm, between about 0.5 μmand about 20 μm, between about 0.5 μm and about 15 μm, between about 0.5μm and about 10 μm, between about 0.5 μm and about 5 μm, between about0.5 μm and about 2 μm, between about 1 μm and about 20 μm, between about1 μm and about 15 μm, between about 1 μm and about 10 μm, between about5 μm and about 20 μm, etc. Thus, the average particle size can be anyvalue between about 0.1 μm and about 30 μm, e.g., about 0.1 μm, about0.5 μm, about 1 μm, about 5 μm, about 10 μm, about 15 μm, about 20 μm,about 25 μm, and about 30 μm.

Optionally, conductive particles that may also be electrochemicallyactive are added to the mixture. Such particles can enable both a moreelectronically conductive composite as well as a more mechanicallydeformable composite capable of absorbing the large volumetric changeincurred during lithiation and de-lithiation. A largest dimension of theconductive particles is between about 10 nanometers and about 100microns. All, substantially all, or at least some of the conductiveparticles may comprise the largest dimension described herein. In someimplementations, an average or median largest dimension of theconductive particles is between about 10 nm and about 100 microns. Themixture may include greater than 0% and up to about 80% by weightconductive particles. The composite material may include about 45% toabout 80% by weight conductive particles. The conductive particles canbe conductive carbon including carbon blacks, carbon fibers, carbonnanofibers, carbon nanotubes, graphite, graphene, etc. Many carbons thatare considered as conductive additives that are not electrochemicallyactive become active once pyrolyzed in a polymer matrix. Alternatively,the conductive particles can be metals or alloys, such as copper,nickel, or stainless steel.

For example, graphite particles can be added to the mixture. Graphitecan be an electrochemically active material in the battery as well as anelastic deformable material that can respond to volume change of thesilicon particles. Graphite is the preferred active anode material forcertain classes of lithium-ion batteries currently on the market becauseit has a low irreversible capacity. Additionally, graphite is softerthan pyrolytic carbon, in certain directions of force, and can betterabsorb the volume expansion of silicon additives. Preferably, a largestdimension of the graphite particles is between about 0.5 microns andabout 100 microns. All, substantially all, or at least some of thegraphite particles may comprise the largest dimension described herein.In some implementations, an average or median largest dimension of thegraphite particles is between about 0.5 microns and about 100 microns.The mixture may include about 2% to about 50% by weight of graphiteparticles. The composite material may include about 40% to about 75% byweight graphite particles.

The composite material may also be formed into a powder. For example,the composite material can be ground into a powder. The compositematerial powder can be used as an active material for an electrode. Forexample, the composite material powder can be deposited on a collectorin a manner similar to making a conventional electrode structure, asknown in the industry.

In some embodiments, the full capacity of the composite material may notbe utilized during use of the battery to improve battery life (e.g.,number charge and discharge cycles before the battery fails or theperformance of the battery decreases below a usability level). Forexample, a composite material with about 70% by weight siliconparticles, about 20% by weight carbon from a precursor, and about 10% byweight graphite may have a maximum gravimetric capacity of about 2000mAh/g, while the composite material may only be used up to a gravimetriccapacity of about 550 to about 850 mAh/g. Although, the maximumgravimetric capacity of the composite material may not be utilized,using the composite material at a lower capacity can still achieve ahigher capacity than certain lithium ion batteries. In certainembodiments, the composite material is used or only used at agravimetric capacity below about 70% of the composite material's maximumgravimetric capacity. For example, the composite material is not used ata gravimetric capacity above about 70% of the composite material'smaximum gravimetric capacity. In further embodiments, the compositematerial is used or only used at a gravimetric capacity below about 50%of the composite material's maximum gravimetric capacity or below about30% of the composite material's maximum gravimetric capacity.

Pouch Cell

As described herein, a battery can be implement as a pouch cell. FIG. 1shows a cross-sectional schematic diagram of an example of a Li-ionbattery 300 implemented as a pouch cell. The battery 300 comprises ananode 316 in contact with a negative current collector 308, a cathode304 in contact with a positive current collector 310, a separator 306disposed between the anode 316 and the cathode 304. A plurality ofanodes 316 and cathode 304 may also be arranged into a stackedconfiguration with the separator 306 separating each anode 316 andcathode 304. Each negative current collector 308 may have one anode 316attached to each side; each positive current collector 310 may have onecathode 304 attached to each side. The stacks are immersed in anelectrolyte 314 and enclosed in a pouch 312. The anode 302 and thecathode 304 may comprise one or more respective electrode films formedthereon. The number of electrodes in the battery 300 may be selected toprovide desired device performance.

With further reference to FIG. 1, the separator 306 may comprise asingle continuous or substantially continuous sheet, which can beinterleaved between adjacent electrodes of the electrode stack. Forexample, the separator 306 may be shaped and/or dimensioned such that itcan be positioned between adjacent electrodes in the electrode stack toprovide desired separation between the adjacent electrodes of thebattery 300. The separator 306 may be configured to facilitateelectrical insulation between the anode 302 and the cathode 304, whilepermitting ionic transport between the anode 302 and the cathode 304.The separator 306 may comprise a porous material, such as a porouspolyolefin material. However, the separator material is not particularlylimited.

The Li-ion battery 300 may include an electrolyte 314, for example anelectrolyte having a composition as described herein. The electrolyte314 is in contact with the anode 302, the cathode 304, and the separator306.

With continued reference to FIG. 1, the anode 302, cathode 304 andseparator 306 of the Li-ion battery 300 may be enclosed in a housingcomprising a pouch 312. In some embodiments, the pouch 312 may comprisea flexible material, so it may readily deform upon application ofpressure on the pouch 312, including pressure exerted upon the pouch 312from within the housing. For example, the pouch 312 may comprise alaminated aluminum pouch.

In some embodiments, the Li-ion battery 300 may comprise an anodeconnector (not shown) and a cathode connector (not shown) configured toelectrically couple the anodes and the cathodes of the electrode stackto an external circuit, respectively. The anode connector and a cathodeconnector may be affixed to the pouch 312 to facilitate electricalcoupling of the battery 300 to an external circuit. The anode connectorand the cathode connector may be affixed to the pouch 312 along one edgeof the pouch 312. The anode connector and the cathode connector can beelectrically insulated from one another, and from the pouch 312. Forexample, at least a portion of each of the anode connector and thecathode connector can be within an electrically insulating sleeve suchthat the connectors can be electrically insulated from one another andfrom the pouch 312.

A Li-ion battery comprising an electrolyte composition as describedherein, and an anode having a composite active material film asdescribed herein, may demonstrate reduced gassing and/or swelling atroom temperature (e.g., about 20° C. to about 25° C.) or elevatedtemperatures (e.g., up to about 85° C.), increased cycle life at roomtemperature or elevated temperatures, and/or reduced cellgrowth/electrolyte consumption per cycle, for example compared to Li-ionbatteries comprising conventionally available electrolyte compositionsin combination with an anode having the same active material. In someembodiments, the Li-ion battery as described herein may demonstratereduced gassing and/or swelling across various temperatures at which thebattery may be subject to testing, such as temperatures between about−20° C. and about 130° C. (e.g., compared to the same Li-ion batteriescomprising conventionally available electrolyte compositions).

Gaseous byproducts may be undesirably generated during batteryoperation, for example, due to chemical reactions between theelectrolyte and one or more other components of the Li-ion battery, suchas one or more components of a battery electrode. Excessive gasgeneration during operation of the Li-ion battery may adversely affectbattery performance and/or result in mechanical and/or electricalfailure of the battery. For example, undesired chemical reactionsbetween an electrolyte and one or more components of an anode may resultin gas generation at levels which can mechanically (e.g., structuraldeformation) and/or electrochemically degrade the battery. Thus, thecomposition of the anode and the composition of the electrolyte can beselected to reduced gas generation.

Various embodiments have been described above. Although the inventionhas been described with reference to these specific embodiments, thedescriptions are intended to be illustrative and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

What is claimed is:
 1. An energy storage device comprising: a firstelectrode; a second electrode; a separator between the first electrodeand the second electrode; and an electrolyte system comprising: adihydrofuranone based compound; a linear carbonate; a cyclic carbonate;and a Li-containing salt.
 2. The energy storage device of claim 1,wherein the dihydrofuranone based compound is of the formula (I):

wherein R₁ is a C1-C8 alkyl substituted by F; and wherein each R₂, R₃,and R₄ is independently an —H or a C1-C8 alkyl.
 3. The energy storagedevice of claim 1, wherein the dihydrofuranone based compound isselected from the group consisting ofalpha-(Trifluoromethyl)-gamma-butyrolactone andγ-Methyl-α-(trifluoromethyl)-γ-valerolactone, or combinations thereof.4. The energy storage device of claim 1, wherein the linear carbonate isselected from the group consisting of ethyl methyl carbonate (EMC),dimethyl carbonate (DMC), and diethyl carbonate (DEC).
 5. The energystorage device of claim 1, wherein the electrolyte system furthercomprises a co-solvent selected from the group consisting of methylacetate (MA), ethyl acetate (EA), methyl propanoate, and gammabutyrolactone (GBL).
 6. The energy storage device of claim 1, whereinthe cyclic carbonate is selected from the group consisting offluoroethylene carbonate (FEC), di-fluoroethylene carbonate (DiFEC),Trifluoropropylene carbonate (TFPC), ethylene carbonate (EC), vinylcarbonate (VC), propylene carbonate (PC),-fluoromethyl-5-methyl-1,3-dioxolan-2-one (F-t-BC),3,3-difluoropropylene carbonate (DFPC), and3,3,4,4,5,5,6,6,6-Nonafluorohexyl-1-ene carbonate.
 7. The energy storagedevice of claim 1, wherein the cyclic carbonate is a fluorine containingcyclic carbonate.
 8. The energy storage device of claim 7, wherein thefluorine containing cyclic carbonate is FEC at a concentration of about5% or more.
 9. The energy storage device of claim 1, wherein theelectrolyte is substantially free of non-fluorine containing cycliccarbonate.
 10. The energy storage device of claim 1, wherein theLi-containing salt is a fluorinated Li salt.
 11. The energy storagedevice of claim 1, wherein the Li-containing salt has a concentration ofabout 1 M or more.
 12. The energy storage device of claim 1, wherein atleast one of the first electrode and the second electrode is a Si-baseelectrode.
 13. The energy storage device of claim 12, wherein theSi-based electrode is an anode.
 14. The energy storage device of claim13, wherein the anode is a Si-dominant anode.
 15. The energy storagedevice of claim 13, wherein the anode comprises: greater than 0% andless than about 99% by weight of Si particles, and greater than 0% andless than about 90% by weight of one or more types of carbon phases,wherein at least one of the one or more types of carbon phases is asubstantially continuous phase that holds the composite material filmtogether such that the silicon particles are distributed throughout thecomposite material film.
 16. The energy storage device of claim 15,wherein the anode comprises greater than 50% and less than about 97% byweight of Si particles.